1. Field of the Invention
The present invention relates to a steering apparatus for a vehicle having a variable transmission ratio of a steer angle of turned wheels to a steering angle of a steering wheel.
2. Related Background Art
In the comparative steering apparatus, rotation of a steering shaft coupled to the steering wheel is normally converted to lateral displacement of a tie rod through a gearing system such as rack and pinion. In order to assure running stability according to vehicle speed on this occasion, the apparatus is provided with a transmission ratio changing mechanism for changing a rotation transmission ratio (gear ratio) between input and output of the gearing system in accordance with vehicle speed.
FIG. 9 illustrates an example of the comparative steering apparatus provided with such a transmission ratio changing mechanism which is similar to a mechanism described in Japanese Laid-open Patent Application No. 62-23869. In this steering apparatus, rotation of steering shaft 102 coupled to the steering wheel is transmitted through input gears 103, 104 to input shaft 101, whereby the input shaft 101 rotates in unison with sun gear 105. The rotation of the sun gear 105 is transmitted through planetary gears 106, disposed on the outside periphery of the sun gear, to ring gear 107, and the apparatus is structured to gain rotational output from output shaft 108 which is integral with the ring gear 107.
On the other hand, rotation of stepping motor 109 is transmitted to carrier 110, which is rotatable relative to the input shaft 101, and the carrier 110 supports the planetary gears 106.
In this setup, when the carrier 110 is rotated by the stepping motor 109, the planetary gears 106 come to revolve around the sun gear 105. This action changes an amount of rotation of the planetary gears 106 rotated by the sun gear 105. The rotation transmission ratio between the input shaft 101 and the output shaft 108 is controlled by this mechanism.